This invention relates to an actuator, and in particular to an actuator of the type known as a hole in the wall (HITW) actuator.
FIG. 1 illustrates, diagrammatically, a typical HITW actuator which comprises a housing defining a cylinder 1 within which a piston 2 is slidable. The piston 2 divides the cylinder 1 into first and second chambers 3, 4, each of which communicates with a respective passage 5, 6. The passages 5, 6 are connected, in use, to an appropriate valve arrangement whereby the chambers 3, 4 are supplied with fluid under either low or high pressure. The effective area of the piston 2 exposed to the fluid pressure within the first chamber 3 is substantially equal to that exposed to the fluid pressure within the second chamber 4. It will be appreciated that by applying fluid under high pressure to one of the chambers and fluid under low pressure to the other of the chambers, the piston 2 can be moved to and held in an end position relative to the housing. By applying fluid under high pressure to a chamber which was at low pressure, and by venting the chamber which was at high pressure, the piston 2 can be moved to and held in an opposite end position.
A rod 8 is secured to the piston 2 such that movement of the piston 2 results in extension or retraction of the rod 8 relative to the housing.
Approximately mid way along the length of the cylinder 1, a third passage 7 known as a hole in the wall is provided. Depending upon the position of the piston 2, the third passage 7 can communicate with either the first chamber 3 or the second chamber 4 or may be closed by the piston 2.
In use, with the third passage 7 isolated from both the high and low pressure sources, the actuator operates as described hereinbefore. In a further mode of operation, the third passage 7 is connected to a source of fluid under low pressure, and both the first chamber 3 and the second chamber 4 are supplied with fluid under high pressure. With the piston 2 in its right hand position as shown, the fluid pressure within the first chamber 3 will be lower than that within the second chamber 4 as the third passage 7 communicates with the first chamber 3. As a result, the piston 2 will move towards the left, movement continuing until the piston 2 reaches a position in which it covers the third passage 7. When the third passage 7 is closed, the pressures within the first and second chambers 3, 4 become equal and so no net force is applied to the piston 2 by the fluid. Once this position is reached, all three connections to the cylinder 1 can be broken and the piston 2 will remain in this position. If the piston 2 and rod 8 are subject to buffeting, such buffeting forces will be absorbed by the fluid within the first and second chambers 3, 4 with very little movement of the piston 2 occurring.
According to the present invention there is provided an actuator comprising a piston slidable within a cylinder, the piston defining with the cylinder a first chamber and a second chamber, the effective cross-sectional area of the piston exposed to the fluid pressure within the first chamber being greater than that exposed to the fluid pressure within the second chamber, first and second ports whereby fluid can be supplied to the first and second chambers, respectively, and a third port located intermediate the first and second ports, the piston and the third port being cooperable to throttle the rate at which fluid is able to escape from the first and second chambers through the third port, in use.
In use, where the actuator is controlled in such a manner that the third port is connected to a source of fluid at relatively low pressure, the first and second ports being supplied with fluid at high pressure, the piston will move towards and be held in a position in which the third port communicates with the first chamber, the position of the piston being such that fluid is able to escape from the first chamber at a sufficiently high rate that the pressure within the first chamber is different from that within the second chamber, compensating for the difference in the effective areas of the piston exposed to the fluid pressures within the chambers and in the relatively high magnitude externally applied loads.
In order to achieve the necessary control over the rate at which fluid is able to escape from the first chamber, the piston is conveniently provided with a seal arrangement, forming a seal between the piston and the cylinder, the seal arrangement defining a metering edge which cooperates with the third port to throttle the rate of fluid flow to the third port. The metering edge is conveniently defined by part of a member carried by the piston and formed of aluminium bronze or PEEK.